Posterior and anterior tooth restoration is typically accomplished by excavating a tooth that has decayed or is otherwise in need of repair to form a cavity. This cavity is filled with a paste material, which is then compacted and shaped to conform to the original contour of the tooth. The paste is then hardened, typically by exposure to actinic light. The paste material is a tooth colored, packable, light curable, polymerizable restorative composition comprising a highly filled material.
Posterior tooth restorations, especially the Class II type, require the use of a matrix band for proper application of a restorative. The restorative has to be condensable. That is, as it is packed into the cavity of a tooth surrounded by a matrix band, the restorative must deform the matrix band in such a way that the original tooth contour is achieved. In addition, proper deformation of the matrix band leads to appropriate contact with the adjacent teeth.
Up to now, the only type of restorative having adequate rheological properties for use with a matrix band has been amalgam. Amalgams have been employed as restoratives for this purpose for a long time and they are known to have good wear characteristics, good marginal quality over time due to buildup of corrosion products at the border of the restoration and a small coefficient of thermal expansion. The metallic color, however, is a drawback for their use as is the uncertainty of the biological interactions of the metallic components of dental amalgams.
Tooth colored dental restorative composites are usually composed of dispersions of glass filler particles below 50 gm in methacrylate-type monomer resin. Splintered pre-polymerized particles, which are ground suspensions of silica in pre-polymerized dental resins, may also be used. Additives such as pigments, initiators and stabilizers have also been used in these types of composites. Because the glass particle surface is generally hydrophilic, and because it is necessary to make it compatible with the resin for mixing, the glass filler is treated with a silane to render its surface hydrophobic. The silane-treated filler is then mixed with the resin at a proportion (load) to give a paste with a consistency considered usable, that is to allow the paste to be shaped without it flowing under its own weight during typical use. This paste is then placed on the tooth to be restored, shaped and cured to a hardened mass by chemical or photochemical initiation of polymerization. After curing, the mass has properties close to the structure of a tooth.
Although it has been found that increasing the load of a resin-based composite leads to higher viscosity, amalgam-like condensability has not yet been achieved. There is thus a need in the dental profession to have a resin-based restorative that is condensable and compatible with the use of a matrix band.
As stated previously, the resins typically used in dental restorative compositions are mostly comprised of dimethacrylate monomers. These monomers vitrify quickly upon initiation of polymerization by crosslinking. The added glass particles after polymerization give a higher modulus to the system and reduce crack propagation by dispersion reinforcement.
A significant disadvantage in the use of methacrylate resin-based restorative composites is that they shrink significantly after cure. For example, a modern hybrid composite shrinks approximately 3% after cure. This shrinkage leads to further tooth decay because bacterial infiltration is possible. To address the problem of tooth decay, adhesives are used to coat the tooth surface to be restored before the application of the composite. The shrinkage stress during the initial phase of the vitrification process, however, is significant and on the order of 1 MPa or higher during the first 20 seconds of light exposure for a light cure composite. This initial stress development compromises the performance of the adhesive. So even with the use of an adhesive, significant marginal breakdown can occur, leading to bacterial infiltration. This process is defined as microleakage and is usually measured by dye penetration methods. Thus, there is also a need to make available to the dental profession a resin-based composite that has reduced volumetric shrinkage and shrinkage stress.
The coefficient of thermal expansion of the glass fillers used in resin-based composites is much closer to tooth structure than that of the resins. So it is desirable to limit the amount of the resin in a dental composite and maximize the amount of filler material. The main factor limiting the volume fraction (load) of the inorganic filler in highly filled suspensions is particle-particle interactions. Dispersants, through their ability to reduce interactions between particles can improve the flow (reduce the viscosity) of the suspension, therefore allowing a higher load. Dispersants in non-aqueous systems reduce particle interactions by a steric stabilization mechanism. A layer of the dispersant is adsorbed on the surface of the particles keeping them apart from one another, reducing the viscosity. The dispersant structure must contain a chain that allows for steric stabilization in the resin and it also must be strongly adsorbed on the particle surface. There is thus a further need to provide a dispersant that will be effective with a non-aqueous, highly filled suspension containing polymerizable groups for use in a dental restoration.
An additional critical area needing improvement in dental restorations is the wear and abrasion resistance of polymeric restorative compositions. For posterior restorations, the main wear mechanism is generally classified as the three body type, involving food bolus. For anterior restorations, wear is generally classified as the two body type, involving toothbrush abrasion, for example. Wear is caused by the heterogeneous nature of dental composites, occurring mostly through "plucking" of the filler particles from the surface followed by abrasion of the softer resin phase. Because wear in these systems is highly dependant on friction, friction reducing additives are expected to improve abrasion resistance. For example, in Temin U.S. Pat. No. 4,197,234, polytetrafluoroethylene powder or another similar polyfluorocarbon resin or polyfluorochlorocarbon resin is added for improvement of abrasion resistance in a chemically cured dental composite. The polytetrafluoroethylene additive or other similar additives, however, also act as an opacifying agent, making the restoration nonaesthetic. In other words, the color of the restoration does not blend sufficiently with the surrounding dentition. In addition, when the opacity is high, light cure initiation cannot be used. Similarly, Fellman et al. U.S. Pat. No. 4,433,958 describes the use of several fluoropolymers as solid particulate insoluble in the liquid monomer system in dental restorative formulations. Again, highly opaque materials are obtained. There is thus an additional need to provide a dental restorative composite with superior wear and abrasion resistance in both posterior and anterior applications, without causing undue opacity in the restorative.
In summary, the dental profession is in need of a dental restorative that has improved shrinkage properties, higher load capabilities and superior wear and abrasion resistance, and that is condensable and compatible with the use of a matrix band.